My guess is that it is because of the screen ratio of the two phones, since it is just being "mirrored." The iPhone 7 has a 16:9 screen ratio, while the Xs has a 19.5:9 screen ratio -- since the 7 has a "wider" screen, it causes it to be "taller" on your car's display. Looking at the pictures, even the iPhone 7 doesn't fully fill the display on the car, the ratio on the car's screen appears to be about 3:2. I think if you compare the picture to what is displayed on the two phones, you'll find that what is displayed on the phone screen is accurately represented on the car display.